Lighting plays an integral role in cinema, television, theater, painting and still photography. It is the art of manipulating the light for optimum effect. In the case of video and photography, you need strong light to capture an image. However, lighting involves much more than merely supplying the necessary light for an image or scene.
Function of Lighting
Lighting's primary---but by no means only---role is to influence what the audience understands of a given scene or image. Lighting, for instance, can direct the viewer's attention, taking advantage of the human eye's propensity to look at the brightest areas of a frame, according to "The Filmmaker's Handbook" by Steven Ascher and Edward Pincus. Lighting also serves as a storytelling tool, for example establishing setting by indicating the time of day or even the season when a given scene is taking place.
Establishing Mood
The way light affects the way an image looks depends primarily on the quality and distribution of the light, according to Kris Malkiewicz and M. David Mullen's book "Cinematography." These qualities can have a profound effect on the overall "mood" of a scene or image. For centuries, painters such as Rembrandt were aware of the power achieved with the subtle manipulation of light, creating dramatic effects like the "chiaroscuro" style. The advent of motion picture film only re-emphasized the importance of lighting design is order to achieve a desired effect.
Characteristics of Light
Light sources can be described as hard or soft. The hardness or softness of a given light source partly depends on its size and its distance from the subject. Hard light usually travels directly from the filament of the bulb (assuming it's an incandescent light source) to the subject with a minimum of materials in between, like for instance a lens. This results in shadows with sharply defined edges. However, if you place a translucent substance, like a piece of silk, that diffuses the light, it results in weaker, less sharp shadows; hence, "softer" light.
Lighting on Film
The proper choosing and placement of lights on a film set is a high priority for the film crew. Actors must look their best, while the director and especially the cinematographer---also known as the director of photography---strive to achieve a certain "look" or mood to the scene. For that reason, the careful positioning of lights usually consumes a lot of time and expense before cameras start rolling.
Creating Depth
Another important function of lighting is to render an optical illusion; namely, to recreate a three-dimensional reality into a two-dimensional image on your screen. A variety of techniques can achieve this, from lighting prominent subjects from behind (a technique called backlighting) to even simpler methods like leaving parts of the background in shadow.
